Roof moss: when it matters and when it does not

Moss has no roots in the way a tree does. It grips with fine hair-like threads into whatever texture it can find — the weathered face of a concrete tile, the grain of a sanded ridge, the gritty edge of an old slate — and it takes its water straight from the rain and the air. That is why it always picks the same places: the north slope, the side under next door's sycamore, and the bottom two or three courses where run-off lingers.

A pitched roof does not keep water out by being sealed. It keeps water out by shedding it quickly and then drying. Each tile overlaps the one below by a set head lap, and that lap works because the gap between the two drains and dries between showers. Lay a saturated mat across it and that part of the roof stops drying. The tile stays wet, the lap stays wet, and the underlay beneath is asked to do full time a job it was only ever meant to do occasionally.

What that costs you depends entirely on the material. Concrete tiles carry their colour in a thin cement-rich skin on the face; kept permanently damp, frost lifts that skin away in flakes, and what is left underneath drinks water faster than the tile you started with. Clay is denser and largely shrugs it off. Slate does not absorb water at all, so on a slate roof the green is very close to being just green.

Almost every moss-related repair worth doing is a drainage job. The mat sheds clumps in wind and heavy rain, they wash down the slope, and they stop at the first thing that will hold them: the gutter, the hopper head, the valley, or the outlet on a flat roof below.

A packed gutter does not only overflow at the front where you would see it. It overflows at the back too, into the gap between the gutter and the fascia, and that water runs onto the wall head and the feet of the rafters — the timbers you can least afford to keep wet and the last ones anyone ever looks at. Fascia boards rot from behind for exactly this reason, which is why a soft fascia is so often the first visible sign of a moss problem that started two storeys higher.

Valleys are worse, because the water has nowhere sideways to go. On a London butterfly roof — two slopes falling into a central lead-lined valley hidden behind the front parapet — the entire roof drains through one channel and one outlet. Block that and the water rises until it passes the upstand, and it goes into the roof space rather than down the front of the house where somebody would notice.

The verge is the third place to look: the mortar edge at a gable end. That bedding is what holds the edge tiles down against wind lift. A permanent damp strip of moss along it keeps the mortar wet through every frost until it loses its grip and comes away in lengths. Once the verge is open, wind gets under the edge of the slope, and wind lift takes far more tiles off London roofs than weight has ever pushed in.

Most of this can be judged from a first-floor window and from the pavement with a pair of binoculars. Two questions settle it: what the roof is made of, and where the moss has been ending up.

This is the part worth reading twice. A jet wash strips a roof in an afternoon, and it is the most common way a cheap clean turns into a repair bill.

The first problem is the surface. A domestic pressure washer delivers well over a hundred bar at the nozzle; a roof tile was designed to survive rain landing on it. The cement skin on a concrete tile comes off under a lance in seconds, colour and all, and the rougher, more open face left behind grows moss back quicker than the one you started with. On slate the lance breaks the fragile bottom edges and works old nails loose.

The second problem is the direction of the water. A roof relies on gravity and overlap. Aim a lance uphill and water goes straight past the head lap onto the underlay — and on most London roofs built before the 1980s that underlay is bitumen felt, by now brittle and quite likely already split over the rafters. That is how a clean produces a ceiling stain a fortnight later, long enough afterwards that nobody connects the two. Several tile manufacturers exclude pressure-washing damage from their warranties, which tells you what the trade makes of it.

Price is the giveaway. If one quote for a whole roof is a fraction of every other, it is a jet wash, and it is often followed by an offer to spray a coating over what the lance has just done. Hand scraping is slower and dearer because somebody has to be up there working course by course.

The order of work matters more than people expect. The roof is looked at first from a ladder, then scraped and soft-brushed off a roof ladder, which spreads the load across the ridge so nobody stands on the tiles themselves. Gutters, hoppers, valleys and outlets are cleared afterwards rather than before, because that is where everything you have just removed has gone. Tiles the mat has lifted go back mechanically fixed. Anything found broken underneath should be photographed and priced on its own line, not folded into the total.

A biocide goes on last, once the roof is clear and dry enough to take it. It kills the spores and film no brush reaches, and it works over weeks rather than minutes — the dead residue then weathers off with the rain across the following months. A roof still looking patchy a fortnight after the work is behaving normally. It is a treatment, not a coating, and it changes nothing about how the roof sheds water.

Across London this work is priced by roof area rather than by the hour. Four things move the price: access first — a rear slope that needs a tower rather than a ladder adds to the cost — then the thickness of the mat, then the area and pitch, since a 40-degree slope carries about a third more surface than its footprint suggests, and last the London extras such as a parking suspension for the van, which belongs on the quote rather than on the final invoice.

A slate roof with a green cast and some grey-orange lichen does not need cleaning. It needs looking at every few years for slipped slates and tired flashings, which is a different job and a cheaper one. Cleaning it is a decision about how the house looks, and there is nothing wrong with that as long as it is priced as such.

If the only real symptom is a gutter that overflows each autumn, the honest job is a gutter, hopper and valley clear at a fraction of a full roof clean. That is the sentence a lot of roof-cleaning leaflets are written to avoid, and it is true on a good number of the roofs we are called to look at.

If water is coming in now, moss is unlikely to be the cause and a clean is not the answer. A leak wants finding first — a slipped tile, a split flashing, a failed valley lining — and the clean, if it is needed at all, comes afterwards. And a coating sprayed over the tiles after a clean is worth refusing: it seals a tile that needs to dry, bridges the laps so a single tile can no longer be lifted for a repair, and does nothing about the nails and the underlay, which are what actually bring a tiled roof to an end.